Frame Check / Reset Loop / Start Screen / No Repeat

I just have a few simple questions of things I'm having problems with. I am using AS3 on Flash CS4.

1. what is the correct code to check for when it's on a certain frame. In this case, I want it to play the movie until it gets to a certain frame, and then go to and play another part of the clip.

2. Is there a command to tell a loop (in this case a while loop) to reset itself, as in go back to the beginning of it, if a certain condition is met?

3. How do you make it freeze on one frame until you press a start button to have it start playback? I've made the button, but when I put anything other than Stop(); in the actions window whatsoever, it doesn't stop at all.

4. How do you get the clip to play just once and then stop? At default it seems to just keep repeating forever.